The present invention relates to a transmitter for transmitting orthogonal signals, comprising a power amplifier having a transfer characteristic in which gain is substantially independent of applied bias voltage. In the present specification the term transmitter is to be understood to cover a transmitter forming part of a transceiver. Also, while the present specification describes a transmitter for an Orthogonal Frequency Domain Multiplexing (OFDM) system, it is to be understood that such techniques are equally applicable to other systems transmitting orthogonal signals, for example Code Division Multiple Access (CDMA).
OFDM, also known as MultiCarrier Modulation (MCM) or Discrete MultiTone modulation (DMT), is a technique by which data is transmitted at a high rate by modulating several low bit rate carriers in parallel, rather than one high bit rate carrier. OFDM is spectrally efficient, and has been shown to be effective for high performance digital radio links. Application areas include: Wireless Asynchronous Transfer Mode (WATM), for high speed, short distance radio links between computer systems; Digital Audio Broadcasting (DAB), for high quality audio signals; Microwave Video Distribution System (MVDS); and future mobile radio systems such as Universal Mobile Telecommunication System (UMTS).
An important characteristic of a Radio Frequency (RF) signal for transmission is the crest factor, defined as the ratio of the peak value of an AC waveform to its Root Mean Square (RMS) value. In an OFDM system the crest factor can be high since it is possible for the signals in each of the sub-channels to be in phase (giving rise to a peak value that is the product of the number of channels and the amplitude of the signal in each channel), but on average the phases will be randomly distributed (giving rise to a much lower mean value). For example, in a 16 carrier OFDM system the peak power can be 16 times the mean transmission power.
The need to be able to transmit signals with such a high crest factor places considerable design constraints on the transmitter""s power amplifier, in particular its linearity to meet requirements for permissible levels of compression and clipping. Generally such an amplifier has a poor DC to RF power conversion efficiency which may result in the generation of excessive amounts of heat and which also has a detrimental effect on battery life if the amplifier is incorporated in portable equipment.
The crest factor can be reduced by preventing the combination of certain phase modulation states from being applied to the carriers. However, this has the disadvantage that more symbols need to be transmitted for a given amount of data as each symbol has fewer available states. Such techniques are well known, one example being a xc2xe rate scheme for a four carrier OFDM system, which reduces the crest factor from 4 to 1.9. U.S. Pat. No. 5,636,247 describes a more sophisticated technique of this type. When applied to a 16 channel system a crest factor reduction of 3 dB can be achieved using a {fraction (13/16)} rate scheme.
An alternative method is described in U.S. Pat. No. 5,610,908, in which a number of closely spaced carriers are modulated (in this case using QPSK) and then transformed to the time domain by an Inverse Fast Fourier Transform (IFFT), as is usual. The signals are then limited and transformed back to the frequency domain by a Fast Fourier Transform (FFT) where phase and amplitude adjustments may be made to some of the signals, and then transformed back to the time domain with an IFFT. From here the transmission proceeds as normal. An example is given of a 2048 channel OFDM system for which a simulation of twenty random signals, initially having a crest factor of 9.38 dB, demonstrated that the crest factor could be reduced to 3.4 dB.
It can be seen that although the techniques outlined above can reduce the crest factor they cannot reduce it to unity (corresponding to a constant envelope modulation). Hence, there are still significant constraints on the design of power amplifiers for use in OFDM systems which limit the efficiency of DC to RF power conversion.
An object of the present invention is to maximise the DC to RF power conversion efficiency of an orthogonal signal transmitter.
According to the present invention there is provided a transmitter for transmitting orthogonal signals, comprising a power amplifier having a transfer characteristic in which gain is substantially independent of applied bias voltage, means for determining the maximum crest factor in an interval of a signal to be transmitted and means for varying the DC bias voltage applied to the amplifier in response to the determined crest factor of the signal.
A known method by which the efficiency of a power amplifier that is subject to varying power levels can be improved is to vary the DC bias conditions to suit the applied signal level. If the signal level is low a low bias voltage is applied, while if the signal level is high a high bias voltage is applied. Providing the gain of the amplifier is invariant, little or no distortion should occur. This can either be done by switching between different supply voltage levels depending on the signal amplitude (the resulting amplifier being known as a class G amplifier), or by amplitude modulating the supply voltage to keep it at a level which is just sufficient to amplify the instantaneous peak signal level without distortion (the resulting amplifier being known as a class H amplifier).
The present invention is based upon the recognition, not present in the prior art, that in a transmitter including a power amplifier having the characteristic of substantially constant gain irrespective of applied DC bias voltage, the bias voltage of the amplifier can be controlled with reference to the crest factor of a signal to be transmitted to improve the DC to RF power conversion efficiency of the transmitter.